1. Field of the Invention
The present invention relates to ink jet printing and more particularly to an improved droplet exciter used in continuous or Rayleigh-type ink jet printers.
2. Prior Art
A variety of ink jet printing architectures are known in the art. Each of these architectures can be used to selectively encode a recording member such as paper or the like with patterns of ink droplets in a controlled manner. So called drop on demand printing, for example, is practiced by causing a recording member to pass in the vicinity of a printer from which ink droplets are generated only at specified intervals. While the so called drop on demand type printing has certain advantages, most specifically the simplicity of design of the ink jet printer, it is questionnable if drop on demand type printers can achieve speed and resolution requirements deemed necessary for a high quality printer.
A second type of ink jet printing is so called continuous type printing wherein ink droplets are continuously generated by a drop generator but wherein only selected ones of those generated drops strike the recording member. Each known type of continuous printer presently employs some type of gutter or catching mechanism to intercept droplets as they proceed toward the print medium to allow a pattern to be created on that medium. The continuous type printer operates due to the phenomenon, first recognized by Lord Raleigh, that when a liquid column is squirted through an orifice, the liquid breaks up into individual droplets due to surface tension effects within the ink column. At the point of droplet breakoff, continuous ink jet printers can selectively induce a net charge on the ink droplets so their trajectory toward the recording member can be controlled with an external electric field to either strike the gutter or the paper.
The ink droplets must form at definite distances from the droplet generator so the charging electrodes can be located at the droplet break off point. To control the point of droplet formation so called droplet exciters are used in the ink jet printing art. These exciters set up perturbations or pressure waves in the ink to control both the location of droplet formation and the size of droplets which are formed. U.S. Pat. Nos. 4,282,532 and 4,296,417, both of which were filed June 4, 1979 and have been assigned to the assignee of the present invention, relate to droplet excitation method and apparatus for enhancing droplet formation. The subject matter of both patents concerns thin droplet exciters using polyvinylidene difluoride (PVF.sub.2) as the droplet excitation material. The subject matter of those two prior art patents is expressly incorporated in this application by reference. According to the disclosure of these patents, a thin film of PVF.sub.2 is positioned against an interior face of a rigid wall of an ink jet fluid chamber and coupled to a source of energization which periodically drives the droplet exciter so as to cause droplet formation at a well defined distance from that generator as ink is squirted from the one or more ink jet orifices in the drop generator. These steps insure that a charging electrode fixed in relation to the generator can induce controlled charges of an appropriate magnitude and polarity on ink droplets as they are formed. While functioning properly at low excitation levels, the operation of thin film PVF.sub.2 drivers has yet to be tested with a high speed printer.
As the resolution of the ink jet printer is increased (i.e. the number of ink droplets per inch rises above 200 spots per inch, if the printing speed is to be maintained) the frequency with which ink droplets must be generated also increases. For example, one proposal for ink jet printing with a resolution of 600 spots per inch operates with a droplet generator drive frequency of about 370 KHz. To achieve this high speed, high resolution printing, it is thought to be desirable that no satellite droplets are formed as the ink droplets break off from the ink columns.
Satellite droplets are small ink droplets which form at the time the main ink droplets used for printing are generated. These satellite droplets must merge with the main droplets prior to contact with the print medium or the satellite droplets would form their own individual printed regions on the paper that would degrade image quality. For low resolution, low speed printing, the satellites present no problem since they merge prior to striking the paper. For high speed printing, however, the satellite droplet formation should be avoided and one way of doing so is to increase the drive level or perturbation provided by the droplet excitation member.
PVF.sub.2 is not an efficient enough driver to provide the needed perturbation at low excitation voltage levels and to excite PVF.sub.2 strongly enough to avoid satellite formation would require application of a signal which would exceed the breakdown strength of PVF.sub.2. Theoretical calculations also indicate that even if the PVF.sub.2 could withstand the additional drive signal, so much heat would be dissipated during printer operation that special steps would need to be taken to assure uniform performance as that heat is added to the printing system. When the drive level of any droplet exciter is increased to avoid satellite formation, the energy dissipation in the form of heat rises as a quadratic function of the drive voltage. Energy dissipation in the form of heat generation in the ink can change the point of droplet formation thereby exacerbating the problem the droplet exciter was used to solve.
The perceived difficulties with PVF.sub.2 drive materials have suggested other drivers be examined. In particular, a material which exhibits stronger piezoelectric activity at excitation levels lower than the breakdown field of the material is advised. Certain ceramics exhibit high piezoelectric activity and have been tested as droplet exciters with some success. The major problems associated with the use of ceramic drivers are that they are brittle and thus prone to breakage during handling and that large area flat sheets of ceramic piezoelectrics are extremely difficult to fabricate. Abutting smaller ceramic plates is unattractive because, again, the brittle ceramic must be handled and because the edge surfaces can lead to high dielectric losses (and a concomitant heating problem) and to low dielectric breakdown strength. For these reasons, improved droplet drivers having the requisite drive capabilities and areas are one goal not yet satisfactorily achieved in the ink jet printing art.